The invention relates generally to molecular level cleaning of parts by vapor degreasing. More particularly, the invention relates to a solvent mixture comprising n-propyl bromide, a mixture of low boiling solvents and, preferably, a defluxing and/or ionics removing agent and/or at least one saturated terpene, as well as to a method of cleaning an article in a vapor degreaser using the solvent mixture. The solvent mixture of the invention is non-flammable, non-corrosive, non-hazardous, and has a low ozone depletion potential.
Molecular level cleaning by vapor degreasing has found wide acceptance in industry. In fact, molecular level cleaning by vapor degreasing is a preferred method of cleaning precision parts, such as electronics, machined metallic parts, etc., since vapor cleaning leaves virtually no residue on the parts. Generally, vapor degreasing involves the heating of a solvent to its boiling point to generate a vapor layer into which the object to be cleaned is placed. The vapor condenses on the object and subjects the surface to a solvent-flushing action as it flows downward. The solvent-flushing action dissolves the hydrocarbon contaminants and removes them from the object, thereby cleaning it. The liquid drops are then collected in a reservoir and are revaporized, typically through the use of steam-heating coils. Thus, the surface of the object is continually rinsed with fresh solvent.
There are four general types of vapor phase degreasers. The simplest form of a vapor phase degreaser is the straight vapor cycle degreaser which utilizes only the vapor for cleaning. As the parts are lowered into the hot vapor, the vapor condenses on the cold parts and dissolves the surface oils and greases. The oily condensate drops back into the liquid solvent at the base of the tank. The solvent is evaporated continuously to form a vapor blanket. Since the oils are not vaporized, they remain in the bottom of the tank in the form of a sludge. The scrubbing action of the condensing vapor continues until the temperature of the part reaches the temperature of the vapor whereupon condensation ceases, the part appears dry, and it is removed from the degreaser. The time required to reach this point depends on the particular solvent employed, the temperature of the vapor, the weight of the part, its specific heat and the type of contamination material to be removed. This particular vapor phase degreaser does an excellent job of drying parts after aqueous cleaning and before plating and, thus, it is frequently used for this purpose in the jewelry industry. Unfortunately, however, it is not as effective on small, light weight parts because such parts frequently reach the temperature of the vapor before the condensing action has fully cleaned the parts.
A second type of vapor phase degreaser, i.e., the vapor-spray cycle degreaser, is frequently used to solve the problems associated with the straight vapor cycle degreaser. In this vapor-spray cycle degreaser, the part to be cleansed is first placed in the vapor zone as is done in the straight vapor cycle degreaser. A portion of the vapor is condensed by cooling coils and fills a liquid solvent reservoir. This warm liquid solvent is pumped to a spray nozzle that can be used to direct the solvent on the part, washing off surface oils and cooling the part, thereby cleaning by vapor condensation.
The third type of vapor phase degreaser is a liquid-vapor cycle degreaser which has one compartment with warm solvent and another compartment with a vapor zone. This degreaser is particularly useful for heavily soiled parts or for cleaning a basket of small parts that nest together. The fourth type of vapor phase degreaser is the ultrasonic degreaser. Such degreasers are useful for cleaning critical parts. An ultrasonic degreaser has a transducer mounted at the base of the tank which operates in the range of 20 kHz to 40 kHz. The transducer alternately compresses and expands the solvent forming small bubbles which, in turn, cavitate or collapse on the surface of the part. This cavitation phenomenon disrupts the adhering soils, thereby cleaning the part.
Conventional solvents used with the foregoing vapor phase degreasers include trichloroethylene, perchloroethylene, methyl chloroform, methylene chloride, CFC 113, dibromomethane, bromochloromethane, trichlorotrifluoroethane and various hydrochlorofluorocarbons, such as xe2x80x9cGenesolvexe2x80x9d (manufactured by Allied Chemical). Vapor degreasing techniques employing the foregoing solvents or equivalents thereof are taught in U.S. Pat. No. 3,881,949 which issued on May 6, 1975 to Carl Martin Brock. Unfortunately, however, such solvents are typically on the Clean Air Act list of high ozone depleting chemicals and, thus, they are being phased out of production and/or banned from use in the United States. Thus, there exists a need for a solvent which can be used in place of these banned ozone depleting chemicals in vapor phase degreasers.
U.S. Pat. No. 4,056,403, which issued to Robert J. Cramer, et al. on Nov. 1, 1977, describes a method in which a number of non-regulated ozone depleting chemicals, including n-propyl bromide, are used in cleaning polyurethane foam generating equipment. Cramer, et al. teach a method wherein a solvent composition described therein is used for cleaning a polyurethane foam generating apparatus or a segment thereof. The solvents taught may be periodically injected under pressure through the mixer portion of the foaming apparatus in order to purge it of residual unreacted or partially foam forming materials. Unfortunately, the method described in this patent would be totally ineffective because its composition does not include the appropriate stabilizers necessary to prevent the n-propyl bromide from becoming acid and thereby attacking the metal surfaces which might be placed into the vapor layer.
The use of hot saturated vapors of a liquid halogenated hydrocarbon, including bromochloromethane, is taught in U.S. Pat. No. 4,193,838 which issued to Robert J. Kelly, et al. on Mar. 18, 1990. More particularly, this patent teaches the generation of a pool of hot saturated vapors of a halogenated acyclic hydrocarbon. Pieces of paper stock which have been coated with xe2x80x9chot meltxe2x80x9d coatings, such as are used on consumer items and milk cartons, etc., are then placed in the vapor pool and, thereafter, they are agitated. Again, it is noted that this method would be ineffective at cleaning flux and other articles because of the acidic nature of the nonstabilized compound utilized therein which would tend to destroy the object rather than just clean it.
U.S. Pat. No. 5,403,507, which issued to Richard G. Henry on Apr. 4, 1995, discloses a solvent mixture for use in vapor cleaning degreasing. Dibromomethane is used as the principal component. The dibromomethane is mixed with other solvents which are intended to stabilize the dibromomethane and to prevent the solvent mixture from becoming acidic on the release of bromine into the atmosphere. Although the solvent mixture disclosed therein is more stable than either of the solvent mixtures taught in U.S. Pat. Nos. 4,056,403 and 4,193,838, there are still a number of disadvantages associated with the use of dibromomethane which make it unsuitable for use as a solvent in vapor phase degreasers. In fact, the Clean Air Act now lists dibromomethane as an ozone depleting chemical which is banned from use in vapor degreasers or any other cleaning process which results in atmospheric release.
In view of the foregoing, it is readily apparent that there remains a need in the art for a solvent mixture which is suitable for molecular level cleaning of parts without the use of any of the high ozone depleting chemicals that are identified as Class I or Class II materials in the U.S. Federal Register, Vol. 58, No. 236, Friday, Dec. 10, 1993, Rules and bromochloromethane as a potential ozone depleter and possible banning in the U.S. Federal Register 40 CFR Part 82, Vol. 60, No. 145, Pages 38729-38734, Jul. 28, 1995.
It is an object of the invention to overcome at least one of the problems described above.
Accordingly, the invention provides a solvent mixture which can be used in vapor phase degreasers in place of traditional solvents.
More particularly, the invention provides a solvent mixture for use in a vapor degreasing system, the solvent mixture comprising effective amounts of n-propyl bromide and at least one low boiling solvent or mixture thereof and, preferably, a defluxing and/or ionics removing additive and/or at least one saturated terpene.
In another aspect, the invention provides a method for cleaning an article in a vapor degreaser, the method comprising: (a) providing a vapor degreaser system; (b) adding to the solvent reservoir of the vapor degreaser system the inventive solvent mixture; (c) boiling the solvent mixture to form a vapor layer; (d) introducing into the vapor layer an article to be cleaned; and, (e) removing the article from the vapor layer.
In this method, the vapor layer condenses on the article, thereby subjecting the surface of the article to a solvent-flushing action as it flows downward. The solvent-flushing action dissolves the hydrocarbon contaminants and removes them from the object, thereby cleaning it. As such, using the method of the present invention, oil, grease, rosin flux and other organic material can be readily removed from the article of interest.
Other features, objects, and advantages of the invention and its preferred embodiments will become apparent from a review of the detailed description which follows.
The solvent mixture of the invention is non-flammable, non-corrosive, and non-hazardous. Moreover, it has a high solvency and a low ozone depletion potential typically between 0.001 and 0.04 and a global warming potential typically between 0.0001 and 0.0003. As such, the solvent mixture of the present invention can effectively be used to remove oil, grease, rosin, flux, and other organic contaminants from the surfaces of numerous articles, e.g., electrical, plastic and metallic parts.
Preferably, the invention provides a solvent mixture for use in a vapor degreasing system, the solvent mixture comprising: (a) about 85 wt. % to less than about 96.5 wt. % n-propyl bromide; (b) 0 wt. % to about 6.5 wt. % of one or a mixture of saturated terpenes, the terpene mixture preferably comprising about 35 wt. % to about 50 wt. % cis-pinane and about 35 wt. % to about 50 wt. % trans-pinane; (c) an effective amount, preferably about 3.5 wt. % to less than about 0.5 wt. % or about 3.5 wt. % to about 5 wt. % of a mixture of low boiling solvents, the low boiling solvent mixture preferably comprising about 0.5 wt. % to about 1 wt. % nitromethane, about 0.5 wt. % to about 1 wt. % 1,2-butylene oxide, and about 2.5 wt. % to about 3 wt. % 1,3-dioxolane; and, (d) 0 wt. % up to about 5 wt. % of at least one defluxing and/or ionics removing additive selected from the group consisting of sec-butanol, ethanol, and methanol.
In a presently preferred embodiment, the terpene mixture of cis-pinane and trans-pinane includes terpenes. Suitable terpenes include, but are not limited to, one or more of the following: endo-isocamphene, xcex1-pinene, cis-para-menthane and trans-para-menthane. In another preferred embodiment, the terpene mixture further includes endo-isocamphene, xcex1-pinene, cis-para-menthane and trans-para-menthane. If present, these terpenes can, individually, make up to 100% of the terpene content of the solvent mixture and, preferably, about 0.01 wt. % to about 6.5 wt. % of the solvent mixture, and highly preferably about 0.05 wt. % to about 5 wt. % of the solvent mixture.
One of functions of the low boiling solvent or low boiling solvent mixture is to neutralize any free acid that might result from oxidation of the mixture in the presence of air, from hydrolysis of the mixture in the presence of water, and from pyrolysis of the mixture under the influence of high temperatures. Moreover, the low boiling solvent or solvent mixture serves to prevent pitting or corrosion of metal articles which are placed in the vapor layer.
It has been discovered that the solvent mixture of the invention meets the desired characteristics for the proper cleaning of electrical parts, metals, plastics, elastomers, circuit boards, etc. More particularly, the solvent mixture of the invention has the following characteristics: (1) it is properly stabilized against any free acid that might result from oxidation of the mixture in the presence of air, from hydrolysis of the mixture in the presence of water, and from pyrolysis of the mixture under the influence of high temperatures; (2) it is non-flammable and non-corrosive; (3) the various components of the solvent mixture are not regulated by the U.S. Clean Air Act; and (4) none of the various components of the solvent mixture are known cancer causing agents (i.e., the various components are not listed by N.T.I., I.A.R.C. and California Proposition 65, nor are they regulated by OSHA). Moreover, the solvent mixture of the invention has a high solvency with a kauri-butanol value above 120 and, more preferably, above 125. In addition, the solvent mixture of the invention has an evaporation rate of at least 0.96 where that of 1,1,1-trichloroethane=1. Upon evaporation, the solvent mixture of the invention leaves a non-volatile residue (NVR) of less than 2.5 mg (or less than about 500 ppm) and, more preferably, no residue. Further, the solvent mixture of the invention has a latent heat of vaporization of about 58.8 cal/g which, in turn, facilitates condensation of the solvent mixture on the chiller side of a standard vapor degreasing system.
In addition, the use of n-propyl bromide in the solvent mixture of the invention has significant advantages over the use of dibromomethane and bromochloromethane. In contrast to n-propyl bromide, dibromomethane is listed by the Clean Air Act as an ozone depleting chemical which is banned from use in vapor degreasing or other cleaning processes involving atmospheric release and bromochloromethane which is suspect of having a ODP of greater  greater than 0.1 where the ODP of the banned 1,1,1-trichloroethane=0.1 and bromochloromethane will not obtain SNAP approval and may be banned. Moreover, in contrast to n-propyl bromide which has an atmospheric life of about 7 to 14 days and an ODP of 0.001-0.04, dibromomethane has an atmospheric life of about three years and bromochloromethane of three to four months and a ODP of 0.08-1.2. Dibromomethane is more toxic than n-propyl bromide and, in contrast to n-propyl bromide, dibromomethane undergoes bioaccumulation (e.g., in fish and aquatic life). In addition, while both dibromomethane and n-propyl bromide react with strong bases, strong oxidizing agents, aluminum, calcium, zinc, magnesium, alloys, etc., the compounds formed with dibromomethane are typically shock sensitive and, thus, potentially explosive, whereas those formed with n-propyl bromide are not. Further, the chemical and physical properties of the n-propyl bromide-based solvent mixtures of the invention make them more energy efficient than the bromochloromethane or dibromomethane solvent mixture of the prior art or the banned solvents 1,1,1-trichloroethylene, trichloroethylene, or methylene chloride. As a result of its boiling point, specific heat and latent heat of vaporization, the n-propyl bromide-based solvent mixtures of the invention require about the same or less energy to cause the mixture to boil and create a denser vapor zone for cleaning.
The sec-butanol, ethanol, and/or methanol present in the solvent mixture function(s) as an aid in defluxing and/or in removing ionic species in vapor degreasing and cold batch cleaning operations.
The presence of sec-butanol enhances flux removal for type R, type RMA, type RA, and synthetic fluxes. Sec-butanol also enhances cleaning of polar and non-polar soils including hand oils, solder oils, greases, silicones, and similar soils.
Methanol is effective in enhancing removal of type A and type RMA fluxes. Ethanol, when present, forms an azeotropic mixture with n-propyl bromide, and is especially useful in defluxing soldered metal parts.
The defluxing and/or ionics removing agent or mixture of agents is typically used in an effective amount of up to about 5 wt. %, preferably 3 wt. % or less, and highly preferably about 1 wt. %, based on the total solvent composition.
As a result of the foregoing properties, the solvent mixture of the invention can be advantageously used in vapor phase degreasers in place of traditional solvents including, for example, trichloroethylene, perchloroethylene, methyl chloroform, methylene chloride, trichlorotrifluoroethane, dibromomethane, CFC-113, etc. Moreover, the solvent mixture of the invention can be effectively used in the four major types of vapor phase degreasers, i.e., the straight vapor cycle degreaser, the vapor-spray cycle degreaser, and the liquid-vapor degreaser and the ultrasonic degreaser. In addition, emissions from a vapor phase degreaser operated with the solvent mixture of the invention are so low that local exhaust ventilation is not required, although in some instances, such a system may still be desirable.
The solvent composition of the invention is simply prepared by combining and mixing together the n-propyl bromide, the terpene mixture (if present), the low boiling solvent mixture, and the defluxing and/or ionics removing agent (if present) in the desired or specified proportions. The solvent mixture is then ready to use as the solvent in a vapor phase degreaser system. n-propyl bromide (C3H7Br, i.e., CH3CH2CH2Br) is commercially available from Dead Sea Bromine LTD Israel. The terpenes used to make up the terpene mixture are commercially available from SCM Glidco Jacksonville Fla. Nitromethane (CH3NO2), 1,3-dioxolane, and 1,2-butylene oxide (also known as 1,2-epoxybutane) are commercially available from Aldrich Chemical Co. (Milwaukee, Wis.). Sec-butanol is also available from Aldrich Chemical Co. In addition to purchasing the foregoing compounds from commercial sources, it will be apparent to those of skill in the art that such compounds can be readily synthesized using known synthetic procedures. For instance, n-propyl bromide can be prepared, for example, when alcohols react with either inorganic acid halides or with hydrogen halides. (see, e.g., Carl R. Noller , Textbook of Organic Chemistry. Ch 6:81 (1956), the teachings of which are incorporated herein by reference for all purposes).
In another aspect, the invention provides a method of cleaning articles in a vapor degreaser using the solvent mixture of the invention. In this method, the solvent mixture of the invention is added to a vapor degreaser, such as Baron-Blakeslee or Branson models. The thermostat on the vapor degreaser is typically set to a temperature of about 156xc2x0 F. to about 160xc2x0 F. (Ultrasonic degreasers can operate effectively at temperatures as low as 70xc2x0 F.) In this temperature range, the n-propyl bromide present in the solvent mixture will boil. When the solvent mixture reaches a temperature of about 156xc2x0 F. to about 160xc2x0 F., a vapor layer will appear above the solvent as a mist. This vapor mist constitutes the principal feature of cleaning by the vapor method. When the vapor mist appears, the object to be cleaned is placed into the vapor layer. The vapor condenses on the object and subjects the surface of the object to a solvent-flushing action as it flows downward. The solvent-flushing action dissolves the hydrocarbon contaminants and removes them from the object, thereby cleaning it. The liquid drops are then collected in a reservoir and are revaporized, typically through the use of steam-heating coils. Thus, the surface of the object is continually rinsed with fresh solvent. As such, using the method of the invention, oil, grease, rosin flux and other organic material can be readily removed from the object of interest. Moreover, the vapors from the solvent will not contain any of the removed contaminants and, thus, the vapors can be used to clean additional objects.